The temperature in the cargo compartment of refrigerated transport vehicles for frozen foods such as fish, meat and ice cream must be maintained below freezing.
U.S. Pat. No. 3,802,212 to Patrick S. Martin et al discloses a refrigeration system which utilizes liquefied cryogenic gas such as liquid nitrogen to control temperature in a cargo compartment in a transport vehicle. Difficulty has been encountered in systems using liquid carbon dioxide as the refrigerant because the temperature in the cargo compartment could not be maintained below approximately 30.degree. Fahrenheit. The carbon dioxide solidified forming dry ice in the system, which required frequent defrosting. Thus, it did not have a commercially acceptable subfreezing capability.
Several patents disclose a back-pressure regulator in a liquid CO2 system between an evaporator and a gas driven motor employed in an effort to modify a liquid nitrogen system of the type disclosed in Martin et al U.S. Pat. No. 3,802,212 to prevent the formation of dry ice in the system by maintaining an operating pressure of 65 psig or higher.
Tyree U.S. Pat. No. 4,045,972 discloses improvements in Martin et al U.S. Pat. No. 3,802,212 including a temperature sensor and a back-pressure regulator installed in an effort to maintain a minimum pressure of, for example, 80 psia to prevent the formation of CO2 snow which could result in blockage or at least a reduced level of operation of the refrigeration system. Three embodiments of the liquid carbon dioxide refrigeration system are disclosed and the disclosure states that the embodiment illustrated in FIG. 4 can be particularly advantageous when it is desired to achieve a cargo compartment temperature of about -20.degree. F. The disclosure states that liquid carbon dioxide is vaporized in a first heat exchanger, passes through a back-pressure regulator and then to a gas driven motor. The gas motor and an expansion orifice are described as being sized so that the temperature drop of the expanding vapor is limited so that carbon dioxide snow is not created.
Tyree U.S. Pat. No. 4,186,562 discloses a liquid carbon dioxide refrigeration system including a back-pressure regulator in the vapor line leading from a vaporizer to maintain a minimum pressure of, for example, 75 psia for the purpose of preventing the formation of snow. The major portion of the vapor stream is described as being expanded through one or more gas motors, passed through one or more additional heat exchangers, and then vented.
U.S. Pat. No. 4,100,759 to Tyree discloses a heat exchanger described as being of sufficient length so that all of the liquid carbon dioxide turns to vapor and exits through a back-pressure regulator that is set to maintain a pressure of at least 65 psig in the heat exchanger coil to prevent the formation of solid carbon dioxide. The carbon dioxide vapor flows 10 through a gas motor drivingly connected to a fan to circulate air in a cargo compartment past the heat exchanger.
The systems using carbon dioxide as a refrigerant have not enjoyed wide spread commercial acceptance because of the tendency of carbon dioxide to solidify and "freeze-up" the system. Recent studies indicate that the atmosphere is being so severely damaged by Freon and other chlorofluorocarbons (CFCs) that their use as refrigerants is being discouraged by governments worldwide. A dire need exists for a refrigeration system which uses a non-polluting refrigerant.